1. Field of the Invention
The present invention relates to an electrolytic capacitor using valve action metal foils, an anode body utilized for an electrolytic capacitor, and method for producing the same.
2. Prior Art
In conventional electrolytic capacitors, valve action metals such as aluminum and tantalum are used as an anode. An oxide film formed on the surface of the metal is used as a dielectric layer to form an anode body. Furthermore, the anode body is applied with liquid or solid electrolyte on the dielectric oxide film, and an anode lead portion and a cathode lead portion are connected to the valve metal and the electrolyte, respectively, the whole capacitor being packaged.
As for the electrolyte of the conventional cathode, an organic solvent, or the like, containing an organic acid, has been used in aluminum electrolytic capacitors, and manganese dioxide in tantalum electrolytic capacitors.
Furthermore, recently, the high-frequency response of electronic parts has increased as electronic circuits have been digitized; and the high frequency response of electrolytic capacitors have been required to improve as well. Proposals have been put forth to use a conductive polymer layer having a high electric conductivity as a cathode electrolyte, to improve the response characteristics by lowering the impedance. Such capacitors have been developed, and put on the market.
On the other hand, as electric and electronic devices are reduced in size, electrolytic capacitors used in them are also required more intensely to have smaller sizes with larger capacitance.
In order to achieve a smaller size and larger capacitance, an anode body formed by laminating a large number of sheets of valve metal foil is also known as an anode body used in electrolytic capacitors. For example, in Japanese Patent Publication JA-A 61-30020, an anode body is disclosed. According to the Publication, sheets of valve metal foil formed so as to have a predetermined size by blanking are laminated. The sheets of valve metal foil are integrated by partially welding some side face of the sheets, and thereafter, the entire laminate is anodized to form a dielectric oxide film on each valve metal layer.
However, the above described method has problems mainly in the following two points. First, if sheets of valve metal foil are laminated and thereafter, anodization is conducted in the state of lamination, then it is extremely difficult to sufficiently anodize all the inner layers of the laminate. As a result, a sufficient dielectric oxide film cannot be formed. If the dielectric oxide film cannot be formed sufficiently, then the capacitance per unit volume in the anode body in an electrolytic capacitor obtained as a final product may be lower than that expected at the time of design, or the breakdown voltage value becomes lower.
Second, in the conventional method, the sheets of metal foil are blanked with a predetermined size. For the requirement of size reduction of electronic parts, therefore, it is necessary to make the laminated metal foils smaller to obtain a small-sized anode body, in accordance with the final capacitor shape. However, there was a limit in production of the low size of valve metal foils and it was further more difficult to laminate such small valve metal foils to anode bodies.
An object of the present invention is to provide an anode body and a method for producing the anode body, in which a sufficient dielectric oxide film can be formed as far as the inside of the anode body formed by laminating sheets of valve metal foil, which is used in an electrolytic capacitor.
Another object of the present invention is to provide an anode body providing a capacitance that capacitors should have, which can be taken out as efficiently as expected from the whole anode body, and a method for fabricating such an anode body.
Another object of the present invention is to provide an anode body for an electrolytic capacitor whose size reduction can be achieved in mass production processes, and a method for fabricating such an anode body.
A further object is to provide an electrolytic capacitor having an excellent high-frequency characteristic with lower impedance, and a method for producing such an electrolytic capacitor.
In a capacitor anode body of the present invention, a fixing frame surrounds the periphery of a laminate including a plurality of sheets of anode valve metal foil, each having a dielectric oxide film formed beforehand on a surface thereof, thereby fixing the laminate. Edges of the anode valve metal foil are exposed on both side faces of the laminate.
In this structure, the fixing frame may be used as an anode conductor by contacting the frame with every valve metal foil in the laminate.
For the anode body, each of sheets of the anode valve metal foil already has the dielectric layer. Therefore, an electrolyte can be impregnated through the edges of exposed side of the laminate into the valve metal foils to form a cathode layer on each of the dielectric layer on the valve metal foils, which is used as an electrolytic capacitor.
The anode body may be employed as an solid electrolytic capacitor by applying a conductive polymer for the electrolyte. In the capacitor, the conductive polymer as cathode layers is formed on the anodized surface of the laminated valve metal foils, which is exposed on the side of the laminate and can contacts with a cathode conductor attached on the side.
The laminate can provide the width between both side faces of the laminate when using rectangular sheets of the anode valve metal foil, i.e., the width of the laminate can be made small. As for the anode body having such a structure, the cathode conductor is applied to the cathode layer exposed on the side face of the laminate. By reducing the width of the laminate, it is possible to shorten electric paths of the cathode layer extending from the dielectric layer of each of sheets of the anode valve metal foil to the cathode conductor. Thus, the internal impedance is lowered in the complete capacitor.
The size of the anode body according to the present invention can be further reduced by making the fixing frame of valve metal, electrically connecting the fixing frame to a metal portion of sheets of the anode valve metal foil, and making the fixing frame serve also as an anode terminal in an electrolytic capacitor of a final product.
In an electrolytic capacitor according to the present invention, the conductive polymer layer is formed beforehand on the dielectric layer which is formed on each of a plurality of sheets of the anode valve metal foil to be laminated. The sheets of the anode valve metal foil are fixed by the fixing frame, resulting in a laminate. A capacitor element is thus obtained, and a cathode conductor and an anode conductor are connected thereto.
In common to the methods of producing an electrolytic capacitor and its anode body according to the present invention, lengthy sheets of anode valve metal foil are laminated, fitted integrally into a groove portion of a lengthy channel-shaped frame. The lengthy laminate is subjected to cutting to yield a large number of anode bodies or capacitor elements. To each anode body or a capacitor element, a cathode conductor and an anode conductor are connected.
A fixing frame surrounds the periphery of a laminate including a plurality of sheets of anode valve metal foil, each having a dielectric oxide film formed beforehand on a surface thereof, and fixes the laminate. On both side faces of the laminate, edges of sheets of the anode valve metal foil are exposed. In the structure of the anode body, each of sheets of the anode valve metal foil already has the dielectric layer. Therefore, when forming a conductive polymer layer on each of the sheets of the anode valve metal foil, it becomes easy to make a monomer to penetrate from the edge portions of sheets of the anode valve metal foil by exposing the monomer to the side face into the laminate, cause polymerization, and thereby form a conductive polymer.